This invention relates to the processing of ball clays.
Ball clay is a sedimentary, kaolinitic clay which has a high elasticity and a particle size distribution such that a substantial proportion of the particles have an equivalent spherical diameter smaller than one micron. Ball clay occurs in the United Kingdom in three relatively small and discrete deposits in North Devon, South Devon and Dorset respectively, and also in the U.S.A., France, Germany, Italy, Spain and other countries. It generally occurs in nature associated with certain impurities, principally with quartz, lignite, mica, feldspar, iron oxides and sulphides. Although the impurities especially the lignite and iron-containing compounds give the clay a colour in its raw state which varies from pale cream to black, when the clay is calcined a good white colour is obtained provided that the iron and titania content is below a certain specified level (the two generally must not exceed 2.2% by weight in toto of the ball clay), since lignite is completely ignited at the high temperatures which prevail in the kilns. The properties of ball clay render it a very desirable ingredient of ceramic bodies and it is also used as a filler for natural and synthetic rubbers.
Conventionally, ball clay is produced by digging the raw clay from the ground in the form of blocks of plastic material which contain about 20% by weight of water. This material may be sold as it is or in a shredded form. Alternatively, the shredded clay may be thermally dried to give a product in lump or pellet form. If desired the thermally dried product may be further dried and disintegrated to powder in, for example, a heated impact mill. Generally speaking no attempts have been made to improve the properties of ball clays, since firstly there have hitherto been ample supplies of good quality ball clay deposits, and secondly it is difficult to process ball clays. Dry refining does not, generally speaking, have a significant effect in improving the properties of ball clays, and wet processing has been ignored because of problems associated with the fine particle size of ball clays.
As reserves of clay having the most desirable properties for the ceramic and other industries are being used up, it is becoming increasingly difficult to meet the demand for clay having these properties. On the other hand, there are large reserves of clay which do not have such desirable properties and for which it is at present difficult to find a market outlet. It is therefore becoming necessary to use these poorer grade ball clays and to develop processes for their improvement.
Our pending British Patent Application No. 47635/73 describes a process of improving the properties of a ball clay comprising the steps of:
a. suspending the raw ball clay in water to form a suspension the solids content of which is not greater than 55% by weight;
b. subjecting the aqueous suspension of raw ball clay thus obtained to a beneficiation operation in order to remove undesirable impurities from the ball clay;
c. thickening the aqueous suspension of beneficiated ball clay in an apparatus, for example a centrifuge, which is equipped with means for discharging a thickened, solid-containing material to produce a dewatered, beneficiated ball clay the solids content of which is at least 50% by weight; and
d. drying the dewatered, beneficiated ball clay.
Prior to the invention described and claimed in British Patent Application No. 47635/73, ball clay had not been processed in the wet state. The proportion of very fine particles (equivalent spherical diameter less than one micron) in ball clay makes filtration of an aqueous suspension of the clay by conventional means exceedingly slow or impossible. This difficulty was obviated in the earlier invention described above by use of a centrifuge. Although this process gives very satisfactory results the thickening step (c) has been found to be expensive. It is an object of the present invention to provide a process for improving the properties of relatively low-grade ball clays, which process does not include an expensive dewatering step.